In recent years, semiconductor devices have been highly integrated and have achieved increased operating speeds. The degree of integration has been greatly improved particularly by LSI and VLSI technologies, for example. Thus, the heat radiation property of a substrate for carrying a semiconductor device has become ever more important.
A ceramic material used as a substrate for such ICs has generally been made from alumina (Al.sub.2 O.sub.3). However, a conventional alumina sintered body has a low thermal conductivity and an insufficient heat radiation property. Thus, it is difficult to use an alumina sintered body as a substrate capable of sufficiently conducting and dissipating the increased heat generated by ever more integrated IC chips.
In place of such an alumina substrate, attention has recently been directed to a substrate or a heat sink made of aluminum nitride having high thermal conductivity.
Aluminum nitride is a material essentially having a high thermal conductivity and high electrical insulability, with no toxicity, unlike beryllium, which also has high thermal conductivity. Thus, aluminum nitride provides an excellent electrical insulating material or a package material for a semiconductor device. However, an aluminum nitride sintered body having the aforementioned characteristics has a poor junction strength between the aluminum nitride and a metal or vitreous material coating on the surface of the aluminum nitride. The coating forming a metallized layer may be applied by several methods such as a thick film forming method of directly applying commercially available metallized paste onto the surface of the aluminum nitride sintered body, or by a thin film method of forming a thin film of an active metal or by a technique such as vapor deposition for forming a metal film. However, these known methods do not provide a sufficient junction strength between the aluminum nitride sintered body and the coating film. In practice, therefore, it is necessary, for example, to reform or treat the surface of the aluminum nitride sintered body by some technique before or during the metallization step, in order to improve the junction strength of a metal coating on the AlN sintered body.
In a well-known conventional method for such reforming, an oxide layer is formed by oxidizing a surface of an aluminum nitride sintered body.
For example, Japanese Patent Publication No. 58-11390 (1983) discloses a method of forming an oxide layer of SiO.sub.2, Al.sub.2 O.sub.3, mullite, Fe.sub.2 O.sub.3 and the like on a surface of an aluminum nitride sintered body. However, while such an oxide layer has an excellent affinity to a glass layer, it is believed that it has a poor or low affinity to an aluminum nitride sintered body, which is disadvantageous because the junction strength is not reliable. More specifically, an unreliable junction strength between the oxide layer and the aluminum nitride sintered body involves no dispersion, the junction strength cannot be maintained constant in a prescribed heat cycle test, etc.
Japanese Laying-Open No. 63-115,393 (1988) discloses a method of applying a conductive paste mainly composed of tungsten and/or molybdenum and an agent for reinforcing the junction strength made of an oxide mixture of SiO.sub.2, Al.sub.2 O.sub.3 and CaO onto an aluminum nitride sintered body and thereafter firing the same at a temperature of at least 1600.degree. C. However, such a firing temperature is undesirably high and the resultant metallized layer is rather insufficient in its reliability as discussed above regarding Japanese Patent Publication No. 58-11390.